Electrical impedance is a very sensitive indicator of minute changes in organic and biological material and especially tissues such as mucous membranes, skin and integuments of organs, including changes due to irritation of caused by different reactions. Therefore, a lot of efforts have been made to find a convenient way to measure variations and alterations in different kinds of organic and biological material to be able to establish the occurrence of such alterations which are due to different states, characteristics or irritations from e.g. diseases. Such disease includes Squamos cell carcinoma (SCC), malignant melanoma, and basal cell carcinoma (BCC), which is the most common skin cancer. Its incidence is increasing in many countries throughout the world. Exposure to ultraviolet light or ionizing radiation increases the risk for developing BCC and other tumours as well as long term immunosuppression in connection with, for example, an allogeneic organ transplantation. There seems to be no apparent genetic connection and in many patients no other predisposing factors have been found. Traditionally, skin tumours, such as malignant melanoma, have been diagnosed by means of ocular inspection by the dermatologist, in combination with skin biopsy. However, clinical diagnosis of skin tumours is proven to be difficult even for experienced dermatologists, especially in the case of pigmented lesions. In the clinic there is thus a need for a diagnostic aid besides the established method of ocular inspection by the dermatologist in combination with skin biopsies for histological examination.
In light of this, significant work has been done in order to develop diagnostic tools for the diagnosis of tumours in the skin based on impedance measurements. In WO 92/06634 a device for non-invasive measurement of electrical impedance of organic and biological material is presented. The device includes a probe having a number of concentric ring electrodes. The electrodes are driven from a control unit in such a way that the electrical current path defining the actual tissue under test is pressed towards the surface of the tissue part under test. By varying a control signal it is possible to select the region to be tested. The capability of a control electrode to vary depth penetration is limited by the shapes, sizes and distances of the electrodes and the dominating factor determining the depth penetration is distance between the electrodes.
WO 01/52731 discloses a medical electrode for sensing electric bio-potentials created within the body of a living subject. The electrode comprises a number of micro-needles adapted to penetrate the skin. The micro-needles are long enough to reach the stratum cornium and penetrate at least into the stratum corneum and are electrically conductive on their surface and connected to each other to form an array. In EP 1 437 091, an apparatus for diagnosis of biological conditions using impedance measurements of organic and biological material is disclosed. The apparatus comprises a probe including a plurality of electrodes, where each electrode is provided with a number of micro-needles each having a length being sufficient to penetrate at least into stratum corneum. The micro-needles according to EP 1 437 091 are also “nail-like”, i.e. they have stem having a substantially circular cross-section with a constant or a gradually decreasing diameter and a tip-portion with a substantially spherical or needle-shaped tip.
However, clinical experience has shown that lesions, especially in early stages, include very small malignant parts, sometimes being of the magnitude down to 1 mm or less. It has further been shown that it is very difficult or almost impossible to identify such small malignant parts of diseased tissues using the prior art methods and devices due to the limited or coarse spatial resolution, both with regard to tissue depth and with regard to a lateral dimension of the tissue (i.e. in tissue layer being parallel with the surface of the skin), in the impedance spectra obtained by means of these prior art methods. It is important to detect the diseased condition, e.g. malignant melanoma, at an early stage, since the prognosis for the patient will be improved significantly since proper treatment can be initiated when the malignant part still is small. Hence, there is an evident risk using the prior art methods that diseased skin conditions such as malignant melanoma at early stage conditions are not observed due to this limited or coarse spatial resolution.
In light of this, there is a need within the art of a device and method that provides an improved spatial resolution, both in a depth dimension and in a lateral dimension, of the obtained impedance spectra in order to enable detection of diseased conditions such as malignant melanoma at an early stage.